Well, there are books on subjects that are widely used in different countries, Sheldon&Ross for probability, Cormen for Algorithms, etc; Usually a professor cho

Well, there are books on subjects that are widely used in different countries, Sheldon&Ross for probability, Cormen for Algorithms, etc; Usually a professor choose his own way to teach things, with different example using maybe his own experience in the field, putting more focus into certain things, and students can just do better with a (usually free) reference of what is done in a particular class We can argue if this method, where not every class on the same subject is equal, is good or bad, but till this is what is widely used, it's good for both students and professor to have a reference.